Experimental Study On Hybrid Coagulation-microfiltration Process For Drinking Water Treatment

The application of membrane technology in water treatment is highly concerned with the stricter standards for drinking water quality, and improving the productivity of membrane process is more important than ever. In this study, a coagulation-microfiltration process was operated to treat the membrane backwash water and membrane clean water to meet Standards for Drinking Water Quality (GB5749-2006). The aim of the experiment was for improving the productivity of the membrane process. In addition, the coagulation-microfiltration process was studied for defluoridation from underground water in this paper. The application field of coagulation-microfiltration process was enlarged.The raw water for the coagulation-microfiltration process was membrane backwash water from a pilot-scale submerged microfiltration unit and membrane clean water from another pilot-scale pressurized ultrafiltration unit, respectively. Two pilot-scale coagulation-membrane systems produced the membrane backwash water and clean water. DOC and trihalomethanes formation potential in membrane backwash and clean water were mainly distributed in molecular weight>30k Da and <1k Da, UV254 was main in molecular weight<1k Da. The quality of membrane backwash and clean water was influenced by the quality changes in the raw water for the pilot experiments during different periods. The quality of membrane clean water was worse as the filtration time was extended.The powdered activated carbon (PAC) was added in the coagulation-microfiltration system to treat the membrane backwash water from submerged microfiltration unit. The concentrations of turbidity, CODMn, TOC, UV254, trihalomethanes, Fe and bacteria in the treated water from the coagulation-microfiltration system were below the limit value of the drinking water standards. The organic matter in the treated water distributed mainly in molecular weight<1k Da, and the effects of coagulation, PAC adsorption and microfiltration on the organic removal were complementary.When the proper coagulant and PAC were added in the system treating the membrane clean water from pressurized ultrafiltration unit, the quality of the treated water meets the drinking water standards. The removal of organic matter in the membrane clean water with the filtration time of 20 min was main in the molecular weight>10k Da, and the removal rate was increased with the increase of the coagulant dosage. The organic matter with molecular weight<1k Da was not removed significantly, and the changes of removal rate with the coagulant dosage had no good correlation. The adding of PAC improved effectively the removal of organic matter and trihalomethanes formation potential in membrane clean water with the filtration time of 30 min with molecular weight<1k Da.The membrane fouling could be alleviated by the flux reduction and the adding of PAC. The cake layer on the membrane surface was the main cause for the specific flux decrease. The organic foulant was the most important fraction in the membrane foulants. The majority of organic foulant had low molecular weight, and Fe was the main element on the fouled membrane surface among the inorganic elements.Aluminum sulfate was more proper than polyaluminum sulphate as the defluoridation agent of underground water. The defluoridation was dramatically effected by the pH in the experiment. For improving the defluoridation efficiency, four methods, which was adding H2SO4 or HNO3, dissolving CO2 and reducing aeration intensity, respectively, were adopted to reduce pH. The results showed that CO2 dosing was much better than the other methods by comparing the treated water quality. The concentration of fluoride was less than 1.0 mg/L, and the quality of the treated water meets the drinking water standards.